(1) Field of the Art
This invention relates to a switching structure having a thermally responsive element made from material such as bimetallic plate, trimetallic plate or the like with a dish-shaped portion formed at the central area, and more particularly concerns a switching structure in which the thermally responsive element is cantilever supported to reverse the inside out and move back with snap action in response to the ambient temperatures for making and breaking contact between movable and fixed contacts.
(2) Description of the Prior Art
In a thermally responsive switching structure of this type there is often employed a snap action thermally responsive relay device. A thermally responsive element comprising a rectangular bimetallic plate having a dish-shaped portion at the central area as seen at numeral 1 in FIG. 1 is employed. The plate 1 has one end rigidly secured to a connector iron 2 by means of welding or the like as seen at the cross mask in FIG. 1. The other end carries a movable contact 3 made from suitable material such as, for example, silver-based alloy.
Such is the construction that the plate 1 reverses with snap action to turn its full lined curvature into the broken lined one when the ambient temperature is elevated to reach, for example, 125.degree. C., and moves back to the full lined original position when the ambient temperature decreases, for example to 80.degree. C.
This is in detail seen at the graph of temperature characteristics in FIG. 3 which shows how the movable contact 3 displaces its specified point (referred to a portion depicted at C) as the ambient temperature rises, employing the axis of ordinates as displacement D of the portion C, while the axis of abscissas as temperature T on the assumption that the plate 1 has its connector iron 2 affixed to a stationary member such as, for example, a frame by a suitable means.
As the ambient temperature rises from the normal temperature T.sub.0, the thermally responsive plate 1 gradually moves at its outer periphery with its curvature remaining unchanged for the time being (this motion is termed as "creeping" hereinafter).
With its creeping movement, the plate 1, of course, displaces its portion C from the reference position D.sub.0 to the position D.sub.2 along the characteristic curve A. However, the curvature is reversed with snap action to move the portion C from the position D.sub.2 to the position D.sub.6 at the temperature of T.sub.3 such as, for example, 125.degree. C.
Upon the application of temperature of above T.sub.3, the plate 1 creeps to increase the displacement of the portion C, however, no further development in regard to the displacement of the portion C is suggested since it has no direct connection with how the thermally responsive relay device works.
With the decrease of the temperature, the thermally responsive plate 1 creeps to displace the portion C along the characteristic curve B toward the position D.sub.4, and moves back to reverse the curvature into the original position so as to displace the portion C from the position D.sub.4 of the curve B to the position D.sub.1 of the curve A at the temperature of T.sub.1 such as, for example, 80.degree. C. Further temperature decrease from T.sub.1 to T.sub.0 functions to return the portion C to the reference position D.sub.0 along the curve A.
The plate 1 displaces with respect to the temperature in a manner thus far described, however, we depict the negative characteristic curve represented by dotted line connecting the curve A at temperature T.sub.3 and the curve B at temperature T.sub.1 which is conveniently termed "unstable region" at the temperature ranging from T.sub.1 to T.sub.3.
In so doing, a support blade 6a is arranged to have its fixed contact 6 brought into engagement with the movable contact 3 to exert pressure in the direction of the curvature at the normal temperature as seen in FIG. 2. This makes it possible for the plate to snap at the temperature T.sub.2, for example, 120.degree. C. without permitting the plate to substantially creep.
To in detail mention the above structure, the plate 1 has the connector iron 2 secured to the horizontal half 4a of an L-shaped frame 4, and the support blade 6a has its one end secured through electrically insulated materials to the vertical half 4b of the frame 4 to form a cantilever support construction as well known for those versed in the art.
This is accounted that the thermally responsive plate 1 snaps to displace the portion C from the position D.sub.3 to the position D.sub.5 each time when the temperature rises from T.sub.0 to reach to T.sub.2 since the portion C is normally biased to occupy the position D.sub.3 by the forcible engagement of the fixed contact 6 against the movable contact 3.
In this instance, the engagement between the fixed contact 6 and the movable contact 3 is maintained until immediately before the plate 1 snaps, thus preventing the contact 3 from chattering upon breaking contact between the former and the latter.
With the decrease of the temperature, the plate 1 creeps along the curve B to occupy the position D.sub.4 at somewhat short of T.sub.1, and moves back with snap action at T.sub.1 to displace the portion C from the position D.sub.4 to the position D.sub.3 for making contact between the contacts 3 and 6.
It is of importance to suggest that the point gap between the contacts 3 and 6 immediately before the plate snaps at T.sub.1 is the minimum available distance equivalent to the difference between the positions D.sub.4 and D.sub.3, while the similar point gap is equivalent to the difference between the positions D.sub.1 and D.sub.4 in those instances where the plate is in free condition with no bias toward the movable contact 3 presented.
It is in general that industrial part components are acceptable as long as they are within a certain tolerance range, so some thermally responsive plates have temperature characteristics as indicated at Ah, Nh and Bh in FIG. 3 deviated from the precedent one among a multitude of thermally responsive plates. Temperature calibration is such that the plate which has the above temperature characteristics makes the portion C position at D.sub.3h as seen from the curve Nh when the plate is on the point of snapping at T.sub.2.
On the other hand, with the decrease of the temperature, the plate creeps along the curve Bh to displace the portion C from the position D.sub.5h to the position D.sub.4h at the temperature T.sub.1h, thus rendering the point gap to be equivalent to so small a difference between the positions D.sub.4h and D.sub.3h.
That is to say, if the plate is calibrated at the snap temperature by selecting the position of the movable contact from the characteristics of displacement D vs. temperature T, the possibility that the point gap when the plate is about to move back becomes unacceptably small.
Take, for example, a thermally responsive plate of 7 mm in width and 12 mm in length, although the dimension of both displacement D and temperature T are exaggerated for clarity in FIG. 3.
The point gap which is equivalent to the difference between the displacements D.sub.4h and D.sub.3h is diadvantageously less than 0.1 mm in contrast to the instance in which the point gap resulted from the difference between the displacements D.sub.4 and D.sub.3 is relevantly around 0.3 mm. In consequence, the isolating voltage between the contacts reduces to such an extent that the plate moves back accompanied by chattering at the contact with the decrease of the temperature, thus rendering unacceptable to incorporate it with a relay device.
One of the problems caused by the above situation results in a low yield rate upon manufacturing product due to a tight tolerance restriction that each thermally responsive plate must have characteristics extremely close to that represented at A, N and B among a multitude of similar plates prepared.